The present invention relates to a method and apparatus which is particularly useful in correcting for base line shift in liquid chromatography during a solvent gradient run when the proportions of constituent solvents are changed.
One of the most useful liquid chromatography techniques has been found to be a solvent gradient scan in which the proportions of constituent solvents are changed in a predetermined program. This is usually carried out while employing an unchanging wavelength of illumination. The process is sometimes referred to as "gradient elution". One of the solvents is usually a weak solvent, and the other a strong solvent, for the sample materials, resulting in a clear separation of the different constituents of the sample. Such techniques, and the strong benefits of such techniques are described, for instance, in INTRODUCTION TO MODERN LIQUID CHROMATOGRAPHY by L. R. Snyder and J. J. Kirkland published in 1974 by John Wiley & Sons, Inc.
In the practice of gradient elution, it has now become common to provide for a programmed change in the proportions of the different eluants (solvents) in order to improved one or more aspects of the system performance in terms of achieving separation of different sample components.
For this purpose, liquid chromatograph machines are now commercially available which can be programmed to provide highly reproducible solvent flow proportions in a predetermined program. A machine of this type which is available, for instance, from the Perkin-Elmer Corporation of Norwalk, Conn. under the model designation Series 3B is capable of being programmed in as many as five different segments, each segment being programmable to provide for a linear variation of solvents with time, or with concave or convex characteristic gradients. All of the segments of the program are completely reproducible.
Descriptions of liquid flow control systems which are especially useful in liquid chromatographs for obtaining reproducible programmable variations of solvents with time are to be found in various U.S. patents including U.S. Pat. No. 4,084,246, issued on Apr. 11, 1978 in the name of the present inventor andassigned to the same assignee as the present invention, U.S. Pat. No. 4,032,445 issued on June 28, 1977 to Minor M. Munk for a LIQUID CHROMATOGRAPHY PUMPING SYSTEM WITH COMPENSATING MEANS FOR LIQUID COMPRESSIBILITY, and U.S. Pat. No. 4,066,879 issued Jan. 3, 1978 to Leaver and Dudley for a MEANS AND METHOD FOR CONTROLLING ELUANT GRADIENT IN LIQUID CHROMATOGRAPHY.
One of the major limitations of the eluant gradient program in the past has been that it is often desirable to use one solvent which has a much higher optical absorbency for the wavelength of light which is desired to be used than does the other solvent. This results in an extreme shift in the base line as the solvent gradient program proceeds. In some instances, the base line shift is so great that it exceeds the sample signals. Without base line correction, this can mean that some desired solvents really cannot be used with certain wavelengths of light for which they are highly absorbent. The uncorrected chromatogram illustrated in FIG. 3 of the drawings, and discussed more fully below, illustrates an example of this problem.
Previous efforts have been made to correct for base line error problems of this kind in similar instruments. For instance, see U.S. Pat. No. 4,084,248 for a METHOD AND APPARATUS FOR ERROR CORRECTION issued to Larkin B. Scott on Apr. 11, 1978 and assigned to the same assignee as the present application. However, the system disclosed in that patent is not particularly adapted to solvent gradient chromatography, and the corrections provided by that patent are limited in slope so that it is questionable whether the system of that patent would adequately correct for the extreme base line errors encountered in solvent gradient chromatography.
Accordingly, it is an important object of the present invention to provide a base line correction method and apparatus which is especially adapted to solvent gradient liquid chromatography and which is capable of providing corrections at a steep slope (rapidly changing signal level) when necessary.
It is another object of the present invention to provide a base line correction method and apparatus which is generally more efficient and more economical and more effective than prior systems of this kind.
Another object of the invention is to provide a base line drift offset correction for all of the base line correction signals for each sample run to compensate for drift of the system output.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.